The most obvious reason to decant is that the wine has thrown a deposit, and that’s really only likely with vintage or crusted ports and aged unfiltered reds. For the process to work effectively the bottle needs to have been upright for several hours, then be carefully poured into the decanter in front of a light (traditionally a candle) so you can see as the sediment inches towards the neck. You need to do this in a single movement so that it doesn’t fall back and get mixed up with the wine again.